Genomic and personalized medicine (GPM) offers the ability to tailor treatments to an individual's unique genomic profile, potentially maximizing effectiveness and minimizing side effects. Despite the promise for GPM to revolutionize care delivery, few studies assess dissemination and impact in community practice beyond small studies from specialized referral centers. Population-based studies are necessary to assess the use and effects of GPM in diverse patient sub-groups and delivery settings. However, the lack of data sources that combine clinical variables, genomic test results, and cancer therapies delivered in both inpatient and outpatient settings has limited the monitoring of the population impact of GPM for persons diagnosed with cancer. To address these gaps in knowledge of GPM in breast cancer, we propose a population-based observational study to assess the recent dissemination of two recently introduced GPM technologies for women under age 65. The first GPM technology is Oncotype DX(R), a gene expression profile assay, which was first incorporated by professional guidelines in 2007. This test is used, along with other prognostic variables, to refine recurrence estimates for selected patient sub-groups to help guide chemotherapy decisions. We will first evaluate patient and health care setting predictors of Oncotype testing; second, we will assess how test results influence chemotherapy use in general practice. Our third aim is to evaluate predictors of a second GPM- based technology, the tailored anti-cancer agent trastuzumab (trade name Herceptin), for women whose tumors fit a specific genomic profile. Trastuzumab has been proven efficacious among women whose tumors over-express the human epidermal growth factor 2 (HER2) proteins, and has been FDA approved and recommended since 2006 for women with HER2 positive, early-stage breast cancer. We will address these 3 aims in an incident cohort of newly diagnosed women under age 65 who are members of WellPoint-affiliated health plans in 5 states. To accomplish our aims, we will create a linked cancer research database consisting of 5 state cancer registries linked with complete insurance claims data, and linked with Oncotype test results. The resulting unique linked database (total cohort n=45,000) will be used to evaluate breast cancer care in our cohort from 2005 through 2010. Our research strategy creates an infrastructure for future comparative effectiveness research on these and other cancer GPM technologies.